Pulmonary surfactant is a complex material composed of several lipids and proteins bound together by non-covalent bonds. Physiological and physicochemical studies show that this material is rapidly metabolized, perhaps in response to physiological requirements, and that either a deficit or excess of this material in the alveolar spaces may be detrimental to good health. Disturbances in its metabolism and function are reflected in the etiologies of certain respiratory and cardiopulmonary diseases. This study will investigate the means by which the metabolism of pulmonary surfactant may be regulated, and measure the turnover rate of this material in canine lung. The study will pay particular attention to the metabolism of the apoproteins found in purified surface active material, utilizing recently developed immunologic and electrophoretic methods. The results of this investigation may provide information on the metabolic fate of both the lipid and protein constituents of pulmonary surfactant, and the means of its physiological regulation. As such, it may be pertinent to the recognition of disturbances in the metabolism of pulmonary surfactant in respiratory and cardiopulmonary disease, possibly providing insight for more effective therapy.